In the processing of substrates, such as semiconductors or display, materials are deposited on the substrate and etched to form electrically conducting interconnects, contacts, and vias. For example, a pattern of electrical interconnect lines can be formed by depositing a metal-containing conductor on the substrate, forming a resist pattern on the conductor, etching the conductor to form the interconnect lines, and then depositing a dielectric layer over the etched interconnect lines. The dielectric layer can be further etched to form contact holes or vias that expose the underlying metal-containing conductor material or other substrate regions, respectively. Electrically conducting material is then deposited into the etched holes to electrically contact the underlying conductor. For example, in the formation of copper-containing interconnects, the dielectric layer can be etched to form contact holes that expose an underlying copper conductor material. A thin seed layer of copper may then be deposited over the exposed copper conductor material and surfaces of the contact hole to facilitate a subsequent copper electroplating process that at least partially fills the contact hole.
However, the metal-containing conductor material can comprise deposits of material that require cleaning before subsequent process steps can be performed. For example, the deposits can comprise a native oxide film that forms when the conductor is exposed to oxygen species during an intermediate process step. A native oxide film often forms in a resist stripping process in which an oxygen-containing gas plasma is used to strip residual resist. The native oxide can also form when transferring the substrate between different process chambers, such as between etching, stripping and cleaning process steps. The native oxide films are undesirable because they increase the electrical resistance at the contact interface between the exposed conductor surface and the subsequently deposited electrically conducting material. The deposits can also comprise other process deposits remaining from previous process steps, such as for example carbon-containing, silicon-containing, fluorine-containing and nitrogen-containing residues. The deposits are also undesirable because they can adversely affect the deposition of the electrically conducting materials onto the exposed conductor surface, for example by forming voids or other irregularities at the interface between the exposed and deposited materials.
The native oxide film can be removed from the metal-containing conductor in a “pre-cleaning” process performed before deposition of the electrically conducting material on the exposed conductor surface. In a typical pre-cleaning process, the exposed surface of the metal-containing conductor is cleaned by an argon plasma that physically bombards the substrate with energized argon ions to sputter off the film. However, it is difficult to determine the correct energy level to be applied to the energized ions. Excessive ion energy can sputter the underlying metal while too low an energy level can cause the film to remain on the substrate. The film can also be cleaned using an energized reducing gas, such as for example hydrogen, which is chemically reacted with the film to reduce the oxides in the film to volatile hydroxyls and water vapor, as described for example in U.S. Pat. No. 6,346,480 to Cohen et al., which is also incorporated herein by reference in its entirety. However, the reducing agent can also have adverse chemical effects on surrounding materials, for example, the hydrogen species can chemically react with exposed metals to form metal hydrides, which again would undesirably affect the electrical conductivity of the exposed portions.
Conventional cleaning processes are particularly unsuitable to clean metal-containing surfaces surrounded by low-k (low dielectric constant) material, such as for example, Black Diamond™, a low-k silicon oxycarbide fabricated by Applied Materials, Inc., Santa Clara, Calif. In such cleaning processes, the cleaning gases react with the low-k material to change their dielectric values. For example, a conventional cleaning process using cleaning gas combinations such as O2, O2/N2, O2/H2O, O2/N2/H2O, O2/CF4 and O2/CF4/H2O can increase the k value of a low-k dielectric from a k value of about 2.7 to a k value as high as about 4.0, which is similar to the k value for silicon oxide. Low-k materials are believed to be particularly susceptible to damage in these cleaning processes at least in part because the ions generated in these processes, and especially “light” ions such as hydrogen and helium ions, can penetrate deeply into the low-k material and damage the structure of the film. As low-k dielectrics are being more frequently used in semiconductor devices to improve their performance and speed, it is desirable to have a process that can effectively clean these substrates without adversely affecting the dielectric constant values.
Accordingly, it is desirable to deposit metal-containing material on a clean electrically conducting surface without forming a native oxide or other contaminant deposits on the surface. It is further desirable to be able to clean the conductor without adversely affecting surrounding materials. For example, it is desirable to clean a native oxide film from a metal-containing conductor without changing the k value of a surrounding low-k dielectric material on the substrate.